The manufacture of semiconductor (silicon) devices such as silicon integrated circuits and charge coupled devices requires that impurities be kept from residing on the surface of such devices. Impurities such as copper, iron and gold are especially troublesome since they form undesirable charge traps at device surfaces, and can diffuse into the silicon bulk to form hot spots for noise sources, which adversely affect device performance.
Devices formed from a silicon wafer employing processing steps generally include:
(a) thermal diffusion of dopants into the silicon; and
(b) the thermal growth of an oxide over regions of the silicon.
Before each thermal step, it is customary to clean the silicon wafer. Typically this is done by: exposing the water to aqueous solutions of ammonium hydroxide and hydrogen peroxide; rinsing, for a first time, the wafer with deionized water and exposing the wafer to aqueous solutions of hydrochloric acid and hydrogen peroxide; and rinsing, for a second time, the wafer with deionized water. Such a technique, while working to clean away trace amounts of impurities, leaves something to be desired, primarily because it requires handling and processing by wet solutions; and because it takes a fair amount of processing time. Also, particulates present in all liquid reagents tend to become attached to the wafers, where they can create shorts and opens on finely patterned structures.
U.S. Pat. No. 4,159,917 discloses a gaseous cleaning process in which nitric oxide and anhydrous hydrochloric acid are used together with a carrier gas of molecular nitrogen in a temperature range of 850 to 1100.degree. C. While quite effective to remove metallic impurities from the wafers, it has a drawback in that an undesired reaction occurs simultaneously with the silicon in which a film of approximately 75 .ANG. thickness containing silicon, oxygen, nitrogen and chloride is formed (silicon oxynitrochloride). This film prevents a more efficient cleaning of silicon surfaces and frequently inhibits subsequent operations, such as thermal oxidation and dopant diffusion.